High peak power 2.8 µm Raman laser in a methane-filled negative-curvature fiber.

We demonstrate a 2.8 µm gas Raman laser in a methane-filled hollow-core negative-curvature fiber with average power of 113 mW, pulse energy of 113 µJ and estimated peak power of 9.5 MW. Raman quantum efficiency of 40% has been reached from the pump source at 1.064 µm to the 2nd order vibrational Stokes at 2.812 µm using 1.8 MPa methane gas. To our knowledge, this is the first high peak power fiber-based gas Raman laser in mid-infrared region and a range of applications in supercontinuum generation, laser surgery, molecular tracing and gas detection are in prospect.

Hollow-core negative-curvature fiber for UV guidance.

UV guiding fibers are highly sought after in laser and spectroscopy applications. Recent advances in hollow-core fiber orient a practical approach for proper UV light delivery sustainable to high-power and long-term irradiation. In this Letter, we report two types of hollow-core negative-curvature fibers (NCFs) in a UV spectral range. Their structures consist of one ring of six small (7.9 µm in diameter) and four big (20.8 µm in diameter) tubes, enclosing a hollow core of similar size (∼15 µm in diameter). The six-tube NCF shows an attenuation level of 0.13±0.01 dB/m at 300 nm. It is capable of delivering 20 ps, 160 µJ pulses at 355 nm with no damage to the fiber facet. The novel four-tube NCF exhibits an attenuation level of ∼0.3±0.15 dB/m at 355 nm. Its fundamental core mode is guided in an intentionally designed "cladding mode mismatching" region. This four-tube design possesses a high degree of down-scalability for deep-UV guidance and has the potential in attaining polarization-maintaining performance.

Characterization of a liquid-filled nodeless anti-resonant fiber for biochemical sensing.

We theoretically and experimentally characterize a liquid-filled nodeless anti-resonant fiber (LARF) that could find versatile applications in biochemical sensing. When a hollow-core nodeless anti-resonant fiber (HARF) is filled with a low refractive index liquid such as water or aqueous solutions in the whole hollow area, it preserves its anti-resonant reflection waveguiding mechanism with attributes encompassing the broad transmission bandwidth in UV, visible, and near IR; the neglectable confinement loss; and the acceptable single-mode quality. In comparison with other forms of hollow fiber, the moderate core size of our ARF allows both a large analyte-light overlap integral and a fast liquid flow rate. Such a LARF platform offers a promising route for creating compact, integrable and biocompatible all-fiber multifunctional optofluidic devices for in-situ applications. A proof-of-concept experiment of Raman spectroscopy using ethanol is presented, and applications in fluorescence spectroscopy, resonant Raman spectroscopy, noninvasive biochemical analysis, and interferometric sensing are in prospect.

Nodeless hollow-core fiber for the visible spectral range.

We report on a hollow-core fiber (HCF) whose fundamental transmission band covers almost the whole visible spectral window, starting at 440 nm. This HCF, in the form of a nodeless structure (NL-HCF), exhibits unprecedented optical performance in terms of low transmission attenuation of 80 dB/km at 532 nm, a broad transmission bandwidth from 440 to 1200 nm, a low bending loss of 0.2 dB/m at 532 nm under 8 cm bending radius, and single-mode profile. When launched to high-power picosecond laser systems at 532 nm, the fiber, exposed to ambient air, could easily deliver an 80 ps, 58 MHz, 32 W average power laser pulse with no damage and a 20 ps, 1 kHz high-energy laser pulse with a damage threshold in excess of 144 µJ at a fiber output. A proof-of-concept experiment on Raman spectroscopy in ambient air shows the significance of this broadband visible guiding HCF for interdisciplinary applications in nonlinear optics, ultrafast optics, lasers, spectroscopy, biophotonics, material processing, etc.

Bending loss characterization in nodeless hollow-core anti-resonant fiber.

We report high performance nodeless hollow-core anti-resonant fibers (HARFs) with broadband guidance from 850 nm to >1700 nm and transmission attenuation of ~100 dB/km. We systematically investigate their bending loss behaviors using both theoretical and experimental approaches. While a low bending loss value of 0.2 dB/m at 5 cm bending radius is attained in the long wavelength side (LWS) of the spectrum, in this paper, we pursue light guidance in the short wavelength side (SWS) under tight bending, which is yet to be explored. We analytically predict and experimentally verify a sub transmission band in the SWS with a broad bandwidth of 110 THz and an acceptable loss of 4.5 dB/m at 2 cm bending radius, indicating that light can be simultaneously guided in LWS and SWS even under tight bending condition. This provides an unprecedented degree of freedom to tailor the transmission spectrum under a tight bending state and opens new opportunities for HARFs to march into practical applications where broadband guidance under small bending radius is a prerequisite.

Ultrafast Molecular Spectroscopy Using a Hollow-Core Photonic Crystal Fiber Light Source.

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump-probe spectroscopy. A critical requirement here is excellent output stability over extended periods of data acquisition, and we show this can be readily achieved. The time-resolved photoelectron imaging technique reveals nonadiabatic dynamical processes operating on three distinct time scales in the styrene molecule following excitation over the 242-258 nm region. These include ultrafast (<100 fs) internal conversion between the S2(ππ*) and S1(ππ*) electronic states and subsequent intramolecular vibrational energy redistribution within S1(ππ*). Compact, cost-effective, and highly efficient benchtop HC-PCF sources have huge potential to open up many exciting new avenues for ultrafast spectroscopy in the ultraviolet and vacuum ultraviolet spectral regions. We anticipate that our initial validation of this approach will generate important impetus in this area.

Optical Amplification in Hollow-Core Negative-Curvature Fibers Doped with Perovskite CsPbBr3 Nanocrystals.

We report a hollow-core negative-curvature fiber (HC-NCF) optical signal amplifier fabricated by the filling of the air microchannels of the fiber with all-inorganic CsPbBr3 perovskite nanocrystals (PNCs). The optimum fabrication conditions were found to enhance the optical gain, up to +3 dB in the best device. Experimental results were approximately reproduced by a gain assisted mechanism based on the nonlinear optical properties of the PNCs, indicating that signal regeneration can be achieved under low pump powers, much below the threshold of stimulated emission. The results can pave the road of new functionalities of the HC-NCF with PNCs, such as optical amplification, nonlinear frequency conversion and gas sensors.

Hollow-core conjoined-tube negative-curvature fibre with ultralow loss.

Countering the optical network 'capacity crunch' calls for a radical development in optical fibres that could simultaneously minimize nonlinearity penalties, chromatic dispersion and maximize signal launch power. Hollow-core fibres (HCF) can break the nonlinear Shannon limit of solid-core fibre and fulfil all above requirements, but its optical performance need to be significantly upgraded before they can be considered for high-capacity telecommunication systems. Here, we report a new HCF with conjoined-tubes in the cladding and a negative-curvature core shape. It exhibits a minimum transmission loss of 2 dB km-1 at 1512 nm and a <16 dB km-1 bandwidth spanning across the O, E, S, C, L telecom bands (1302-1637 nm). The debut of this conjoined-tube HCF, with combined merits of ultralow loss, broad bandwidth, low bending loss, high mode quality and simple structure heralds a new opportunity to fully unleash the potential of HCF in telecommunication applications.